In vitro production of both murine and human immunoglobulin by B (bone marrow-derived) lymphocytes is largely dependent upon the presence of T (thymus-derived) lymphocytes. The manner in which human T cells augment immunoglobulin synthesis by B cells has not been extensively studied although observations in the mouse suggest that an important mechanism involves the capacity of T cells to release soluble "helper" mediators (both antigen specific and nonspecific) capable of influencing B cell function. In order to gain further insight into the basic mechanism involved in antibody formation in man, this study will critically examine the capacity of human T cell-derived mediators to augment in vitro synthesis of immunoglobulin by B cells. Mononuclear leukocytes will be obtained from normal healthy adult donors by Ficoll-Hypaque centrifugation of heparinized peripheral blood. B cells (and monocytes) will be separated from T cells by double rosetting with AET-treated sheep red blood cells and the cellular constituents of each fraction analyzed by measurement of cells bearing markers for B cells (surface immunoglobulin), T cells (rosetting with sheep red blood cells) and monocytes (esterase activity). In vitro production of IgM, IgG and IgA will be measured by radioimmunoassay of 7 day culture supernatants of B cells (and monocytes) incubated alone and in the presence of either intact T cells or supernatants derived from; 1) alloantigen activated cultures, 2) tetanus toxoid sensitized T cells incubated with tetanus toxoid and 3) mitogen (PWM, Con A, PHA) activated T cells. Supernatants capable of replacing the T cell requirement for in vitro immunoglobulin synthesis will be subjected to a number of fractionation procedures (including Sephadex gel filtration, ion-exchange chromatography and polyacrylamide electrophoresis) in order to characterize factor(s) capable of augmenting B cell function. An understanding of regulatory molecules capable of augmenting B cell function not only will shed insight concerning basic mechanisms involved in human antibody formation but may also offer approaches concerning an understanding of immunodeficiency and collagen vascular diseases (e.g., SLE) in which T cell function may be deranged.